Field of the Invention
The present invention relates to a substrate for a flexible display device. More particularly, the present invention relates to a substrate having excellent heat tolerance and surface hardness, and low price, and a flexible display device including the substrate.
Discussion of the Related Art
Among various display devices such as a liquid crystal display (LCD) device and a organic light emitting diode (OLED) display device, a flexible display device having advantages in a thin profile, a light weight, a large dimensional size, a degree of freedom of shape and a curved display has attracted attention, and a substrate replacing a glass substrate that is heavy and fragile has been developed. Various properties such as heat tolerance, flexibility and transparency are required for a substrate used in a flexible display device (which is referred to as ‘a substrate for a flexible display device’ hereinafter).
Recently, as a device formation technology using a transcription (e.g., Patent Document 1: Japanese Publication No. 2012-156523) is commercialized, mass production of a flexible display device has been realized. A process where a series of fabrication steps are performed for a resin for transcription (e.g., polyimide etc.) on a glass substrate and then the resin for transcription is detached from the glass substrate by a method such as a laser ablation, etc. is required for the device formation technology using a transcription. In addition, since the resin for transcription is required to tolerate a process temperature (500° C.) of forming a thin film transistor (TFT), the resin for transcription has a relatively high price. According to the above factors, a flexible display device has a cost higher than the conventional display device using a glass substrate.
To solve the above problems, a TFT formation technology through a printing method has been suggested. Although the process temperature of forming a TFT is drastically reduced by the TFT formation technology through a printing method as compared with a conventional method, a process temperature of at least 150° C. is required to obtain a property of a TFT and a resistance of a line for real utilization.
Since transparent polyimide and aromatic polyether resin have excellent properties such as heat tolerance and transparency, it has been suggested to use a film of transparent polyimide and aromatic polyether resin as a substrate for a flexible display device (e.g., Patent Document 2: Japanese Publication No. 2010-152004). However, since transparent polyimide and aromatic polyether resin have a high price, there is a problem that a fabrication cost of a flexible display device increases.
Although a conventional resin film such as polyethylene terephthalate (PET), etc. has transparency and low price, the conventional resin film has poor heat tolerance even for a TFT formation technology through a printing method. In addition, the conventional resin has problems that the flexible display device may be easily damaged due to low surface hardness and display quality of the flexible display device may be reduced due to high retardation.
Accordingly, a method of dispersing an inorganic particle in a resin film has been considered as a method of improving heat tolerance and surface hardness.
However, it is difficult to uniformly disperse an inorganic particle in a resin film. For example, a resin film including an inorganic particle may be fabricated by forming a resin material including an inorganic particle into a film shape. Since the inorganic particle may agglomerate and/or precipitate in the resin material, it is difficult to obtain a resin film where an inorganic particle is uniformly dispersed. As a result, a portion of the resin film where the organic particle is not dispersed may have insufficient heat tolerance and surface hardness.